Steels with a high manganese content, due to their advantageous characteristic combination of high strength of up to 1,400 MPa on the one hand and extremely high elongations (uniform elongations up to 70% and elongations at break up to 90%) on the other hand, are basically suitable to a special degree for use within the vehicle industry, particularly car manufacturing. Steels, particularly suitable for this specific application, with high Mn-content of 6 wt %.-30 wt %. are known for example from DE 102 59 230 A1, DE 197 27 759 C2 or DE 199 00 199 A1. Flat products fabricated from the known steels have isotropic deformation behavior with high strength and in addition are also still ductile at low temperatures.
However, counteracting these advantages, steels with a high manganese content are susceptible to pitting corrosion and can only be passivated with difficulty. This large propensity, compared to lower alloyed steel, to locally limited but intensive corrosion with the impact of increased chloride ion concentrations makes it difficult to use steels belonging to the material group of highly alloyed sheet steel especially in car body construction. In addition, steels with a high manganese content are susceptible to surface corrosion, which likewise limits the spectrum of their use.
Therefore, it has been proposed to also provide flat steel products, which are fabricated from steel with a high manganese content, with a metallic coating in the way known per se, which protects the steel against corrosive attack. For this purpose, attempts have been made to apply a zinc coating to the steel material electrolytically.
Although the high manganese-alloyed steel strips, coated in this way, are protected against corrosion by the metallic coating applied thereto, electrolytic coating required for this is a relatively costly operation in terms of process-engineering. In addition, there is a risk of hydrogen absorption, which is harmful to the material.
Practical attempts to provide steel strips having a high manganese content with a metallic protective layer through more economically feasible, practicable hot dip coating, apart from the fundamental problems in wetting with the hot metal, particularly as regards adhesion of the coating to the steel substrate, required in the case of cold forming, brought unsatisfactory results.
The thick oxide layer, which arises from the annealing essential to hot dip coating, was found to be the reason for these poor adhesion characteristics. The sheet metal surfaces, oxidized in such a manner, can no longer be wetted by the metallic coating to the necessary degree of uniformity and entirety, so that the aim of total surface area corrosion protection cannot be achieved.
The possibilities, known from the spectrum of steels, highly alloyed but having lower Mn-contents, of improving wettability by applying an intermediate layer of Fe or Ni in the case of sheet steel comprising at least 6 wt %. manganese have not led to the desired success.
In DE 10 2005 008 410 B3 the application of an aluminum layer to a steel strip containing 6-30 wt %. Mn before final annealing prior to hot dip coating was proposed. The aluminum adhering to the steel strip during annealing before hot dip coating of the steel strip prevents its surface from oxidizing. Subsequently, the aluminum layer, as a kind of adhesion promoter, causes the layer produced by the hot dip coating to adhere firmly over the total surface area of the steel strip, even if the steel strip itself, due to its alloying, presents disadvantageous conditions for this. In the case of the known method, the effect during the annealing treatment essential before hot dip coating, of iron diffusing from the steel strip into the aluminum layer, is exploited for this purpose so that in the course of annealing a metallic deposit, substantially consisting of Al and Fe forms on the steel strip, which then bonds intimately with the substrate formed by the steel strip.
Another method for coating high manganiferous steel strip containing by wt %. 0.35-1.05% C, 16-25% Mn, remainder iron as well as unavoidable impurities, is known from WO 2006/042931 A1. In accordance with this known method the steel strip composed in such a way is first cold-rolled and then being subjected to re-crystallisation annealing in an atmosphere, which is reducing in relation to iron. The annealing parameters are selected such that said steel strip is covered on both faces with a sub-layer which is essentially completely amorphous oxide (FeMn)O and additionally with an outer layer of crystalline manganese oxide, the thickness of the two layers being at least 0.5 μm. Investigations have shown that, in practice, steel strip elaborately pre-coated in such a manner also does not have the adhesion to the steel substrate required for cold forming.
As well as the prior art described above, a method for hot-dip coating hot-rolled steel plate, which possesses high tensile strength, is known from the JP 07-216524 A. In the course of this known method the steel plate is first de-scaled, pickled and cleaned. Then it is weakly oxidized in order to produce an iron oxide film, which has a thickness of 500-10,000 Å, thereon. This iron oxide film is subsequently reduced by reduction heating to active metallic iron. The reduction heating is carried out such that selective oxidation of Si and Mn in the steel and concentration of these elements on the surface are avoided. For this purpose, reduction heating is carried out under an atmosphere, whose hydrogen concentration is regulated in the range of 3-25% vol. so that on the one hand it has sufficient reduction capacity for reducing the iron oxide, on the other hand, however, the selective oxidation of Si and Mn does not happen.